The present invention relates to a process for the production of intermediate oxidation products such as benzyl alcohols and/or benzaldehydes having ether linkages. More particularly, the present invention relates to a process for the production of alcohols and/or aldehydes wherein a toluene having an ether linkage or linkages is oxidized in liquid phase with molecular oxygen for selectively oxidizing the methyl group bound to the benzene ring to hydroxymethyl group or formyl group to convert the toluene into the corresponding alcohol and/or aldehyde.
Benzyl alcohol and benzaldehyde derivatives having ether linkages are useful as perfumes, medicaments and starting materials for other fine chemicals. In particular, m-phenoxybenzaldehyde is very important as an intermediate product for agricultural and horticultural agents. However, the production of these compounds in a good yield is extremely difficult. Up to the present, therefore, an industrial synthetic process as contemplated in the present invention wherein the methyl group of a toluene having an ether linkage or linkages is oxidized in liquid phase with molecular oxygen has not yet been developed. At the present time, an oxidation process wherein an oxidizing reagent such as a permanganate is used is exclusively considered to be industrially advantageous (German Pat. No. 1,953,258).
It is generally known that when a methylated aromatic compound is oxidized with molecular oxygen in liquid phase in the presence of a Redox catalyst, the methyl group of the aromatic compound is converted by oxidation into carboxyl group via hydroxymethyl and formyl groups. In this oxidation process, however, the rate of oxidizing formyl group to carboxyl group is much faster than that of oxidizing methyl group to formyl group. Thus, the production of formyl (aldehyde) compounds in a good yield is extremely difficult according to this oxidation process. For this reason, there has not yet been proposed an industrially operable process for preparing formylated aromatic compounds in a good yield by oxidizing methylated aromatic compounds in liquid phase with molecular oxygen.
With a view to overcoming difficulty in conversion of the methyl group bound to the aromatic ring into formyl group, the present inventor has made extensive researches on auto-oxidation of toluene derivatives in liquid phase with molecular oxygen. As a result of the extensive researches, it has now been found that when a toluene having an ether linkage or linkages is subjected to auto-oxidation conducted by the aid of a relatively large amount of a heavy metal salt, especially a cobalt salt as catalyst in the presence of a lower saturated fatty acid and/or an anhydride thereof, an aldehyde is chiefly formed under the reaction condition using excess oxygen but an alcohol is chiefly formed under the reaction condition using insufficient oxygen. However, this process is effective for oxidation of the methyl group of a toluene having an ether group in p-position but is still unsatisfactory for oxidation of the methyl group of a toluene having an ether group in m- or p-position thereof. In case an ether group is present in m- or p-position to the methyl group of toluene, the reaction velocity in oxidation of such toluene derivative is extremely reduced as compared with the case of oxidizing a toluene having an ether group in p-position thereof, so that the reaction is not initiated under low oxygen pressure nor promoted smoothly even under high oxygen pressure unless severe conditions are adopted. As side reactions take place inevitably under such severe conditions, however, decomposition of the ether linkage tends to occur, thus often resulting in interruption of the reaction. Thus, oxidation of the m- and o-compounds is attended by such disadvantages that a slight change in the reaction conditions induces a significant variation in the result of experiments and that the rate of selection to the end product is considerably reduced as compared with the case of oxidizing the p-compounds. In the case of oxidizing the m- and o-compounds, only a very small amount of impurities causes significant variation in the results of experiments. These disadvantages are apparently shown as bad reproducibility in experiments. This phenomenon is noticeable in the case of the m-compounds which are most difficult to oxidize. For instance, the liquid phase oxidation of m-phenoxytoluene under pressure of oxygen gives as the best result 68% in conversion rate and 35 mol% in the rate of selection to m-phenoxybenzaldehyde. In many cases, however, the reaction did not proceed, or if proceeded, the reaction was interrupted at the stage of 10 mol% or less in the conversion rate even if the reactions were carried out under the same reaction conditions. Thus, the rate of selection was less than 10 mol% in the majority of the cases.
Although many studies were made to explain the reason why reproducibility is not good in the oxidation of the m- and o-compounds, the present inventor failed to reach a persuasive conclusion. As a result of further studies on the stable methods for oxidation, the present inventor found that a process wherein cobalt and nickel ions are used as catalyst or wherein cerium ion is allowed to be coexistent with the catalyst is effective for stable oxidation. However, the result of actual experiments for the m- and o-compounds was not so good. In the case of m-phenoxytoluene, for instance, m-phenoxybenzaldehyde was obtained with good reproducibility but only at 20% in conversion rate and 13.about.17 mol% in rate of selection. As a result of the present inventor's further researches on addition of a third component to the catalyst to improve the above process, it has now been found surprisingly that the existence of a very small amount of a bromine ion-supplying substance in the reaction system is very effective for selective conversion of such hardly oxidizable methyl group in the m- and o-compounds into formyl group. The present invention has been accomplished on the basis of the above finding.
A process for the oxidation of hardly oxidizable compounds wherein a bromine compound is added to the liquid phase auto-oxidation system for methylbenzenes where a cobalt salt in acetic acid is used as catalyst is well known as a process for producing terephthalic acid and is now operated in a large commercial scale. However, no information has been reported on the example for remarkably increasing the rate of selection of intermediate oxidation products by adding a very small amount of a bromine compound to such system. According to the present inventor's study, it has been recognized that intermediate oxidation products such as aldehydes can be produced at a high rate of selection by (a) adding a very small amount of a bromine compound to the liquid phase auto-oxidation system for alkyl or aryl ethers of m-cresol under normal pressure where a cobalt salt is used as catalyst and acetic acid is used as solvent and (b) limiting the conversion rate to 60% or less, in particular 50% or less. In case the bromine compound is absent in the reaction system, the reaction does not take place even under the same reaction condition. Even when the reaction is forced to take place by adding a reaction initiator to the reaction system, the rate of selection to the intermediate oxidation products is only 10 mol% or less at a conversion rate of 20.about.30%. Considering the result of above experiments, the effects achieved by the present invention are indeed surprising and are not believable from the common knowledge in this art.